The present invention relates generally to variable frequency drive (VFD) circuits and, more particularly, to a system and method for speed regulation for VFD circuits where an anti-windup control scheme is employed that provides consistent speed response with no overshoot.
One type of system commonly used in industry that performs power conversion is an adjustable or variable frequency drive circuit, which is an industrial control device that provides for variable frequency, variable voltage operation of a driven system, such as an AC induction motor. VFDs include both an AC-to-DC converter and a DC-to-AC inverter, which includes a plurality of switches that are controlled to provide the variable output of the VFD circuits. VFD circuit DC-to-AC inverters are often controlled by proportional-integral (PI) controllers, proportional-integral-derivative (PID) controllers, and the like. However, when such controllers with integrating action are used, a windup phenomenon appears that degrades control performance of the VFD.
The windup phenomenon is caused by the interaction of the integral term of a controller controlling the VFD and the saturation of a motor being controlled by the VFD. The physical limitations of the motor do not allow the motor to reach its ideal speed as determined by the controller. When the motor reaches its maximum speed, the controller feedback loop breaks. The system then runs as an open loop because the motor will remain at its maximum speed independently of the output of the controller. However, the integral term of the controller will continue to integrate the error between the controller output and the motor speed and “wind up” to a large value. This windup phenomenon can cause large overshoot, slow settling time, and instability in the speed response of a motor.
FIG. 1 illustrates a block diagram of a PID controller 10 in which the windup phenomenon could occur. PID controller 10 transforms a speed error signal, e, into a PID controller output, u′, by inputting the speed error signal, e, into a proportional term 12, an integral term 14, and a derivative term 16 and adding the results of the proportional, integral, and derivative actions together in a summation block 17. The proportional term is represented by proportional block, P, and the derivative term is represented by derivative block, D. In the integral term 14, the speed error signal, e, is input into an integral time block 18 to create an integral speed error signal 20, which is integrated by an integrator block 22. The PID controller output, u′, is transformed into a torque command, u, by inputting the PID controller output, u′, into a saturation block 24 to impose upper and lower limits on the PID controller output, u′, according to the limitations of a motor (not shown) being controlled by a VFD (not shown) associated with PID controller 10. PID controller 10 does not, however, include any anti-windup controls to prevent the windup phenomenon, so PID controller 10 may become unstable.
Those of skill in the art have developed anti-windup controls for preventing the windup phenomenon from occurring in VFD controllers. FIG. 2 illustrates a conventional anti-windup PID controller 26. The PID controller 26 includes the same components and operates in the same manner as PID controller 10 (FIG. 1), except for the additional anti-windup controls. The PID controller 26 uses a tracking-back anti-windup method such that the PID controller 26 adds the negative value of the PID controller output, u′, to the torque command, u, in a summation block 28 to create an output error signal 30. The output error signal 30 is multiplied by a tracking time gain 32 to create a tracking-back signal 34. The tracking-back signal 34 is then added to the integral speed error signal 20 in summation block 26 to create a tracking-back integral signal 38 that is integrated by the integrator block 22 to create an integral term output 40.
The integral term output 40 tracks the saturated voltage of the VFD while the PID controller output, u′, does not equal the torque command, u (during the saturation period of PID controller 26). The PID controller output, u′, will converge to the steady-state value of the load torque on the motor when the torque command, u, is no longer saturated. However, the tracking-back anti-windup method still does not provide optimal performance for VFD controllers. More specifically, the convergence period associated with the tracking-back anti-windup method still leads to overshoot and/or slow response times that result in performance degradation.
It would therefore be desirable to provide a system and method for speed regulation for VFD circuits that provides a consistent speed response with no overshoot.